We have had a very long an damaging outage which lasted around 24hrs due to a change in our DNS servers by our registrar. What happened (according to our registrar) was that for some reason their system identified our domain to be due for renewal and changed the authoritative DNS servers to theirs which caused our domain to be forwarded to a temporary holding page. Once we alerted our registrar to the problem it wad already too late as the error had already propagated. Our domain runs out in Jan 2020 so there was absolutely no reason for this change.
Now my question is, how do other businesses protect themselves against these types of outages which are both out of their control and take hours to resolve, inevitably causing the service to be unreachable for a long period?

You can have all sort of fallback schemes with domains, but the name of the DNS server that the domain registrar points to is the single point of failure. Not the actual server, but the server that they say is authoritative. So I'd pick a registrar that will compensate you if they make that sort of mistake.

Having done that, you can then build in redundancy with nameservers, webservers and so on.